1. Field of the Invention
The present invention relates to a force sensing transducer designed for converting an input force such as compressive force or tensile force into a frequency signal. The force transducer is adapted for wide application to a variety of instruments and apparatus including pressure gauges, gravimeters, differential pressure gauges and level meters.
2. Description of the Prior Art
FIG. 1 illustrates the operating principle of a conventional force-to-frequency transducer known heretofore, in which a bar 1 having a uniform cross section is anchored at two ends thereof on a base 10 such as a case. Supposing now that an axial force S exerted toward the center of the bar 1 is applied to each of the stationary ends thereof, the relationship set forth in Equation (1) exists between the axial force S and the lateral vibration frequency f of the bar when 1&gt;K.sub.2 l.sup.2 /EI S: ##EQU1## in which l: length of bar 1
E: longitudinal elastic modulus of bar 1 PA1 I: secondary moment of cross section related to main axis perpendicular to direction of vibration PA1 g: gravitational acceleration PA1 .rho.: density of bar 1 PA1 A: cross-sectional area of bar 1 PA1 S: axial force (compressive force - positive) PA1 K.sub.1, K.sub.2 : constants determined by bar support conditions and vibration mode.
Since the resonant frequency .omega. is represented by 2.pi.f, the axial force S applied is obtainable by measuring the resonant frequency .omega. of the bar 1.
In such a transducer, the conditions for attaining a high-precision force measurement must satisfy the following requirements:
(i) High stability in the frequency f, signifying that the vibrator (bar 1) has a high Q factor.
(ii) Great frequency change rate .DELTA.f/.sigma. per unit stress. ##EQU2##
The goodness G of the above transducer can be expressed as EQU G=Q.multidot..DELTA.f/S.multidot.A (2)
In the structure of FIG. 1, the input axial force S is obtainable by causing the bar 1 to resonate and measuring its resonant frequency .omega.. However, during vibration as shown in FIG. 1B, a reaction force R is generated at each end of the bar 1 anchored to the base 10, and this force is consumed as a loss unless the base 10 is an ideal stationary end, thereby inducing a reduction in the Q factor of the bar 1.